How<Emphasis Type="Italic"> Daphnia</Emphasis> copes with excess carbon in its food

Animals that maintain near homeostatic elemental ratios may get rid of excess ingested elements from their food in different ways. C regulation was studied in juveniles of Daphnia magna feeding on two Selenastrum capricornutum cultures contrasting in P content (400 and 80 C:P atomic ratios). Both cultures were labelled with ¹⁴C in order to measure Daphnia ingestion and assimilation rates. No significant difference in ingestion rates was observed between P-low and P-rich food, whereas the net assimilation of ¹⁴C was higher in the treatment with P-rich algae. Some Daphnia were also homogeneously labelled over 5 days on radioactive algae to estimate respiration rates and excretion rates of dissolved organic C (DOC). The respiration rate for Daphnia fed with high C:P algae (38.7% of body C day^-1) was significantly higher than for those feeding on low C:P algae (25.3% of body C day^-1). The DOC excretion rate was also higher when animals were fed on P-low algae (13.4% of body C day^-1) than on P-rich algae (5.7% of body C day^-1) . When corrected for respiratory losses, total assimilation of C did not differ significantly between treatments (around 60% of body C day^-1). Judging from these experiments, D. magna can maintain its stoichiometric balance when feeding on unbalanced diets (high C:P) primarily by disposing of excess dietary C via respiration and excretion of DOC.     

Plant functional types broadly describe water use strategies in the Caatinga,a seasonally dry tropical forest in northeast Brazil

1. In seasonally dry tropical forests, plant functional type can be classified as deciduous low wood density, deciduous high wood density, or evergreen high wood density species. While deciduousness is often associated with drought‐avoidance and low wood density is often associated with tissue water storage, the degree to which these functional types may correspond to diverging and unique water use strategies has not been extensively tested.
2. We examined (a) tolerance to water stress, measured by predawn and mid‐day leaf water potential; (b) water use efficiency, measured via foliar δ¹³C; and (c) access to soil water, measured via stem water δ¹⁸O.
3. We found that deciduous low wood density species maintain high leaf water potential and low water use efficiency. Deciduous high wood density species have lower leaf water potential and variable water use efficiency. Both groups rely on shallow soil water. Evergreen high wood density species have low leaf water potential, higher water use efficiency, and access alternative water sources. These findings indicate that deciduous low wood density species are drought avoiders, with a specialized strategy for storing root and stem water. Deciduous high wood density species are moderately drought tolerant, and evergreen high wood density species are the most drought tolerant group.
4. Synthesis. Our results broadly support the plant functional type framework as a way to understand water use strategies, but also highlight species‐level differences.

     

Influence of supplemental UV-B radiation on photosynthetic characteristics of rice plants

In a field experiment with rice (Oryza sativa L. cv. Saket 4) grown under ambient and supplemental ultraviolet-B (UV-B) radiation at 20 % ozone depletion, differences in gas exchange, concentrations of photosynthetic pigments, anthocyanins and flavonoids, biomass accumulation, catalase and peroxidase activities, and contents of ascorbic acid and phenol were determined. Decline in photosynthesis was associated with reductions in stomatal conductance and concentrations of photosynthetic pigments. Enhanced UV-B radiation (eUV-B) increased the contents of flavonoid and phenolic compounds in leaves. Peroxidase activity increased and catalase activity was always lower at eUV-B. The total plant biomass decreased at eUV-B.     

Effect of jasmonic acid on in vitro explant growth and microtuberization in potato

The shoot fresh mass, root length and root numbers of two potato (Solanum tuberosum L.) cultivars Favorita and Helanwuhua were increased significantly by the application of 0.2 – 2 mg dm⁻³ jasmonic acid (JA) in the Murashige and Skoog medium. However, the growth of potato explants was inhibited by JA at high concentrations (20 – 50 mg dm⁻³). Chlorophyll content in explant leaves decreased with an increase in the concentration of JA. In leaves treated with 0.2 mg dm⁻³ JA acid peroxidase activity increased, while in the leaves treated with more than 2 mg dm⁻³ JA peroxidase activity decreased. Under the dark, the microtuber numbers, fresh mass and percentage of big microtubers of two potato cultivars were not promoted by the application of 0.2 – 50 mg dm⁻³ JA.     

Function of the 23 kDa extrinsic protein of Photosystem II as a manganese binding protein and its role in photoactivation

The function of the extrinsic 23 kDa protein of Photosystem II (PSII) was studied with respect to Mn binding and its ability to supply Mn to PSII during photoactivation, i.e. the light-dependent assembly of the tetramanganese cluster. The extrinsic proteins and the Mn cluster were removed by TRIS treatment from PSII-enriched membrane fragments and purified by anion exchange chromatography. Room temperature EPR spectra of the purified 23 kDa protein demonstrated the presence of Mn. Photoactivation was successful with low Mn concentrations when the 23 kDa protein was present, while in its absence a higher Mn concentration was needed to reach the same level of oxygen evolution activity. In addition, the rate of photoactivation was significantly accelerated in the presence of the 23 kDa protein. It is proposed that the 23 kDa protein plays an important role in providing Mn during the process of PSII assembly and that it acquires Mn during the light-induced turnover of D1 in the PSII damage-repair cycle and delivers Mn to repaired PSII.     

Protein structure,electron transfer and evolution of prokaryotic photosynthetic reaction centers

Photosynthetic reaction centers from a variety of organisms have been isolated and characterized. The groups of prokaryotic photosynthetic organisms include the purple bacteria, the filamentous green bacteria, the green sulfur bacteria and the heliobacteria as anoxygenic representatives as well as the cyanobacteria and prochlorophytes as oxygenic representatives. This review focuses on structural and functional comparisons of the various groups of photosynthetic reaction centers and considers possible evolutionary scenarios to explain the diversity of existing photosynthetic organisms.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - Rb Rhodobacter - Rp Rhodopseudomonas     

混合盐胁迫对栾树光合生理指标的影响

该研究以2年生栾树扦插苗为材料,采用盆栽实验方法,设置梯度为0(CK)、100(S₁₀₀)、200(S₂₀₀)、300(S₃₀₀)、400(S₄₀₀)和500(S₅₀₀)mmol·L^-1的摩尔质量比1∶1配制成的NaCl和NaHCO₃混合盐溶液,分析混合盐胁迫对栾树幼苗光合作用和叶绿素荧光特性的影响,以探讨栾树对混合盐胁迫的生理适应机制,为栾树在盐碱地区的引种与推广栽培提供理论依据。结果表明:(1)随着混合盐胁迫程度的增加,栾树出现叶片发黄焦枯、卷曲、根系减少、植株死亡等表观症状。(2)随着混合盐胁迫程度的增加,栾树的叶长、叶宽生长量整体呈下降趋势;栾树的叶长、叶宽生长量在S₁₀₀和S₂₀₀处理下较CK变化不显著,但其叶长生长量在混合盐浓度达到300 mmol·L^-1时较CK显著下降,叶宽生长量在混合盐浓度达到400 mmol·L^-1时较CK显著下降。(3)栾树幼苗叶片的叶绿素含量、PSⅡ实际量子产量(Yield)、光化学淬灭系数(qP)和PSⅡ最大量子产量(F_v/F_m)在S₁₀₀处理下较CK无显著变化;当混合盐浓度达到200 mmol·L^-1时,除F_v/F_m外均较CK显著下降;当混合盐浓度达到300 mmol·L^-1时,F_v/F_m较CK也显著下降。(4)随着混合盐胁迫程度的增加,栾树幼苗叶片的净光合速率(P_n)、气孔导度(G_s)、蒸腾速率(T_r)整体呈下降趋势;在混合盐胁迫0~30 d时,S₁₀₀-S₃₀₀处理的栾树幼苗叶片的P_n、C_i、G_s整体变化趋势一致;随着盐胁迫程度增大,P_n、G_s整体呈下降趋势,C_i整体呈上升趋势。研究发现,在100 mmol·L^-1混合盐胁迫下,栾树生长指标和生理指标变化不明显,表现出一定的耐盐性,此时光合作用主要受气孔限制的影响;当混合盐浓度达到200 mmol·L^-1时,栾树幼苗叶片的生长指标、叶绿素含量、光合生理指标整体显著降低,最终导致植株生长受到抑制,此时光合作用的主要影响因素是非气孔限制和光化学活性失活。     

Copepod and microzooplankton grazing in mesocosms fertilised with different Si:N ratios: no overlap between food spectra and Si:N influence on zooplankton trophic level

We hypothesized that the trophic level of marine copepods should depend on the composition of the protist community. To test this hypothesis, we manipulated the phytoplankton composition in mesocosms and measured grazing rates of copepods and mesozooplankton in those mesocosms. Twelve mesocosms with Northeast Atlantic phytoplankton were fertilised with different Si:N ratios from 0:1 to 1:1. After 1 week, ten of the mesocosms were filled with natural densities of mesozooplankton, mainly calanoid copepods, while two remained as mesozooplankton-free controls. Both before and after the addition of copepods there was a positive correlation of diatom dominance with Si:N ratios. During the second phase of the experiment, copepod and microzooplankton grazing rates on different phytoplankton species were assessed by a modification of the Landry-Hassett dilution technique, where the bottles containing the different dilution treatments were replaced by dialysis bags incubated in situ. The results indicated no overlap in the food spectrum of microzooplankton (mainly ciliates) and copepods. Ciliates fed on nanoplankton, while copepods fed on large or chain-forming diatoms, naked dinoflagellates, and ciliates. The calculated trophic level of copepods showed a significantly negative but weak correlation with Si:N ratios. The strength of this response was strongly dependent on the trophic levels assumed for ciliates and mixotrophic dinoflagellates.     

Non-photochemical quenching of Fo in leaves is emission wavelength dependent: consequences for quenching analysis and its interpretation

The effects of light-induced non-photochemical quenching on the minimal F_o, and variable F_v, fluorescence emissions at 690 and 730 nm in leaves were determined. Non-photochemical quenching of F_o, but not F_v, was found to be dependent upon the wavelength of emission, and was greater at 690 nm than at 730 nm. For emission at 730, compared to at 690 nm, approx. 30% of F_o was not affected by non-photochemical quenching processes in leaves of C3 plants; in maize leaves this was found to be approx. 50%. The data indicate that a substantial proportion of the pigments contributing to F_o emission at 730 nm are not quenched by light-induced, non-photochemical quenching processes and that there are large differences in the pigment matrices contributing to F_o and F_v emissions at 730 nm, compared to those at 690 nm. These findings have important implications for the accurate estimation and interpretation of non-photochemical quenching of fluorescence parameters and their use in the calculation of photochemical efficiencies in leaves. Measurements of fluorescence emissions at wavelengths above 700 nm are likely to give rise to significant errors when used for determinations of photochemical and non-photochemical quenching parameters.